The present invention relates to fiber optic probes for spectrophotometry. In particular, the present invention relates to a fiber optic probe optimized for absorbance measurement applications in fluid media where bubbles, particulate matter, measurement sensitivity, stray light rejection, and flow dynamics are a concern. The present invention further relates to automated, multi-channel spectrophotometric measurements that employ multiple fiber optic probes coupled to either multiple single-channel spectrometers, multiplexed single-channel spectrometers, or single multi-dimensional xe2x80x9cimagingxe2x80x9d spectrographs that employ a two-dimensional CCD (charge-coupled-device) array as the detection and measurement element.
Spectrophotometric absorbance measurements are typically performed by measuring the amount of light (I0) that passes through subject media which contains no sample components of interest and then measuring the amount of light (I) that passes through the subject media that does contain the sample component to be measured. The quantity I0 is referred to as the reference or blank light intensity and the quantity I is referred to as the sample intensity. The concentration of the sample component of interest is proportional to the Absorbance (A), where A=xe2x88x92Log10(I/I0).
The terms xe2x80x9cspectrographxe2x80x9d and xe2x80x9cspectrometerxe2x80x9d are often used in reference to the same instrument type. However, a spectrograph is a special case of a spectrometer that uses a stationary grating and has no parts (other than a shutter in some cases) that move during the measurement cycle. Light is instantaneously diffracted horizontally across the surface of a multi-element array detector (CCD or photodiode). Some spectrometers use a motorized grating to scan across the spectrum and direct light through an exit slit onto a single-element detector.
When fiber optic technology is used, light from a source may be transmitted across a light path gap in the subject media by one or more transmitting fibers, and received by one or more receiving fibers which direct the transmitted light to the detection and measurement device. Probes using this approach are referred to as transmittance probes. In a variation of this basic technique, the transmitting and receiving fibers are side by side and a mirror is used to reflect light back through the subject media onto a receiving fibers. This, in effect, doubles the light path gap.
The applications of in situ or remote measurements using fiber optic spectrophotometric probes has increased significantly in step with advances in fiber optic technology, spectrometry and computing hardware, and data collection and processing software. Many commercial systems now support a wide range of in situ spectrophotometric applications which monitor multiple signals from either multiplexed individual spectrometers or single spectrographs employing two-dimensional CCD detector arrays. Only within the last five years have these advances been applied to commercial in situ spectrophotometric monitoring systems targeted to pharmaceutical in vitro dissolution testing.
There have been minimal advances in fiber optic probe design that target the specific needs of pharmaceutical dissolution testing where flow dynamics and particulate interference are primary concerns. The maintenance of constant flow dynamics throughout an in vitro dissolution test is a major concern of pharmaceutical laboratories that are required by law to perform dissolution testing before allowing product dosage forms to be sold. The in vitro dissolution testing must be done in accord with standards and procedures defined by the U.S. Food and Drug Administration and the United States Pharmacopoeia. Published studies have shown that large diameter probes alter flow dynamics and cause observed tablet dissolution rates to be abnormally high.
Fiber optic probes used in all current commercial in situ dissolution testing systems are based on insertion probe designs commonly employed in industrial environments where the probe must be highly rugged and cylindrical. Typically, the transmitting and receiving fibers are side-by-side (parallel) and in the same enclosure which has a uniform diameter over the submerged portion of the probe. The uniform diameter allows the probe to be readily inserted into a reactor, flowing stream, or other vessel where a seal between the probe and vessel walls is required. Dissolution testing and other forms of laboratory-based testing, where fluid media in a wide-mouthed, unsealed vessel is monitored, have no such requirements for extreme ruggedness or cylindrical configuration.
Therefore, an object of the present invention is to fully meet the needs of laboratory-based, in vitro pharmaceutical dissolution testing and other types of laboratory testing with similar requirements.
Another object of the present invention is to provide a probe for sampling fluids which provides a direct light path between a transmitting optic fiber and a receiving optic fiber without the requirement for additional path-altering optic elements such as mirrors or prisms.
Another object of the present invention is to provide a probe for sampling fluids which improves the optic efficiency over earlier designs, allows small-diameter optic fibers and support tubes to be used to reduce flow disturbances in the sample gap area.
Yet another object of the present invention is to provide a probe for sampling fluids which reduces or eliminates probe surfaces that can trap air bubbles or accumulate debris in the sample gap.
The present invention is a fiber optic probe that includes an open, fine-bodied probe structure and an efficient means of transmitting source light through fluid media to one or more receiving fibers. The probe structure offers minimal resistance or disturbance to fluid flow in the sample area and thus minimizes any affect on solution hydrodynamics or turbulence. The features of one embodiment of the invention are an open structure, very low probe displacement volume and surface area, a light path gap that is perpendicular to the longitudinal axis of the probe in the preferred embodiments, and an efficient means of coupling transmitted light to the receiving fiber that employs no internal optic elements or fiber end modifications. The latter feature also ensures that the invention has excellent stray-light rejection characteristics. Because of its simplicity the probe is economical to manufacture.
Elimination of a discrete light reflecting or refracting element such as a mirror or prism increases the efficiency of light throughput for a given fiber size. Thus it becomes possible to employ smaller diameter fibers than would be required when a discrete light reflecting or refracting element is present. This translates to a significant advantage over conventional designs when the present invention is coupled with a multi-channel xe2x80x9cimagingxe2x80x9d spectrograph based on a two-dimensional CCD array detector.
Coupling to the spectrograph is achieved by bringing the distal ends of receiving fibers together into a vertical array bundle that is mounted to the input of a commercially available spectrograph. The spectrograph employs a fixed grating to diffract the light across the wavelength range of interest and additional optic elements to image the multiple light beams onto the surface of a two-dimensional CCD array. Example arrays are composed of 256xc3x97256, 512xc3x97512, 1024xc3x971024, and other variations on detecting pixel configurations. A CCD spectrometer that could formerly support a maximum of six to eight probes using prisms and 600 xcexcm fibers would now be able to support 12 or 18 xe2x80x9ctransverse light pathxe2x80x9d probes of the present invention design using 300 or 200 xcexcm fibers. This translates to a significant advantage in the application of dissolution testing which is done in groups of six to eight vessels. The CCD spectrometer in the previous example would now be able to support up to three dissolution baths or experiments at the same time.
The probe of the present invention significantly reduces the cost of equipment required to automatically monitor multiple dissolution tests by elimination of expensive optic elements in the optic probe and increasing the number of channels which one spectrograph can monitor. Multi-channel instruments can monitor more probes of the current design as compared to conventional probes.
In use, one or more probes are inserted into the reference fluid media and the reference intensities are measured and recorded. If the concentrations of chemical species are to be measured, then the probes will be inserted into solutions containing a known concentration of reference standard chemical. The reference standard absorbance values will be measured and response factors calculated for each probe. These factors will be used to convert sample absorbance values into concentration values. The probes are then rinsed and inserted into the sample fluid media and monitoring of the sample intensities is performed according to the requirements of the particular application.
In the example application of pharmaceutical dissolution testing, the probes are placed into standard, wide-mouthed dissolution vessels from above the fluid media surface. Typically they are held in place by a suitable clamp or holder for the duration of the test, or they may be periodically raised above the sampling point by an automated mechanism for periods of time when measurements are not being made. The latter action may be needed to reduce effects on solution hydrodynamics produced by conventional insertion probes. The clamp that holds the probe structure must be adjustable to allow for different sampling heights that correspond to different solution volumes that may be employed during the dissolution test. The USP standard sampling point is half way between the top of the stirring element (paddle or basket) and the top of the test solution. Thus the distance from the light path gap to the vessel cover will vary depending on the solution volume. The clamp must allow the operator to properly configure the probe xe2x80x9csampling heightxe2x80x9d prior to starting the test.
During the course of sample measurements, bubbles may be produced as a result of liquid media de-aeration. In some applications, as in pharmaceutical dissolution testing of dissolving tablets, particulate matter will also be present during the entire measurement period. The open structure and transverse light path of the present invention helps ensure that particulate matter and bubbles will not be trapped in the light path gap and produce erroneous absorbance measurements. The low displacement volume and surface area of the present design allows the probes to remain in the vessels for the duration of the test and obviates the need for an additional automated mechanism to raise the probes above the sampling point.
Together the different embodiments of the present invention form a family of fiber optic probe designs that can accommodate a wide range of sensitivity or light path length requirements for monitoring a variety of pharmaceutical dosage forms during dissolution tests.